Learning Guide for Chapter 13 - Alkynes

I. Introduction to Alkynes - p 1

II. Natural Occurrence and Uses of Alkynes - p 5

III. Physical Properties of Alkynes - p 7

IV. Spectroscopy of Alkynes - p 7

V. Nomenclature of Alkynes - p 8

VI. Acid-base Properties of Alkynes - p 12

VII. Synthesis of Alkynes by Elimination - p 14

VIII. Reduction of Alkynes to Form Alkenes and Alkanes - p 15

IX. Addition of HX and X2 to Alkynes - p 17

X. Hydration of Alkynes - p 18

XI. Oxidation of Alkynes - p 20

I. Introduction to Alkynes

Classifying Alkynes

Which of these is an alkyne? What is the other one called?

alkyne unsaturated ketone

O

hydrocarbon w/ a C---C not a hydrocarbon, so not an alkyne

Which of these is a terminal alkyne? Which is an internal alkyne?

internal alkyne

C---C between 2 C's

terminal alkyne

C---C on the end

internal injuriesinternal combustion engine

a terminal diseasean airport terminal

Alkenes could also be labeled this way, but they usually aren't. Why not?

alkenes - no change in properties or behavior

alkynes - different IR, acid//base properties, reactivity

Hybridization and bond formation

Where do each of the bonds come from?

C C HH

What hybridization do each of the carbons in acetylene have?

hybridized atomic orbitals: sp + sp + p + p

C-H bond

C-C ! bond

C-C " bonds

s + sp = sigma

sp + sp = sigma

p + p = pi

What kind of molecular orbital forms each bond?

C C HH

sp

What orientation will the pi bonds have to each other? 90o C C HH

show molecular model

C C HH sp spspsp

p

pp

p

s s

How do triple bonds compare to double and single bonds in length?

triple bonds are shorterHC CH H2C CH2 H3C CH3

121 pm 134 pm 153 pm

Geometry

What geometry do carbons in triple bonds have? linear

Draw a 6 carbon alkyne with the triple bond on the end. Then draw a 6 carbon alkyne with the alkyne in the middle.

How big does a ring have to before it can contain a triple bond (and be stable)?

at least 9 carbons

unstable! stable

one more than trans C=C in a ring

LG Ch 13 p 2

cyclooctyne has been isolated by polymerizes at room temp

sp

< <

sigma sigmapi

pisigma

sigma

pi

pi

sigmasigma

C

from the side

wrong right wrong rightnot linear

LG Ch 13 p 3

Stereochemistry

Can the carbon in a triple bond be a stereocenter? no :)

no cis/trans

no up/down

correct - linear geometry

Can alkynes be chiral? yes

xx

other C's in the molecule can be stereocenters

Molecular formula

What is the molecular formula for a simple alkyne? CnH2n-2

What would be the formula of an alkyne with 10 carbons?

What would be the formula of a cyclic alkyne with 10 carbons?

C10H18

C10H16

How many units of unsaturation does a triple bond have? 2

Stabilization

Which is more stable, an internal alkyne or a terminal alkyne? Why?

internalmore stable

terminalless stable

like C=C : more stable = more conjugated

pi bonds overlap neighboring w/ C-H bonds(hyperconjugation)

we'll look at delta H's on p 15

Reactivity

LG Ch 13 p 4

Where are the two places that alkynes can react?

H

pi bonds

H (terminal alkynes)

How can a terminal alkyne react with a base?

H

base + H-base

used in SN2 and E2 reactions

as a strong Nu/base - acetylide ion

How can an alkyne react with an acid? With an electrophile?

weak acid

H-base

E

Which is more reactive, an alkene or an alkyne? alkyne

264 kJ/mol to break pi bond

226 kJ/mol to break pi bond

H

E

How is the product of an alkyne reaction different from the product of an alkene reaction?

alkene:

alkyne:

HCl

HCl

Cl

Cl

HCl

Cl Cl

still a C=C left can react again

1

2

strong

weak base

weak Nu

strong acid

strong E

1

2

LG Ch 13 p 5

II. Natural Occurrences and Uses of Alkynes

Alkynes are rarely found in nature. What is one exception?

diacetylene

found in carbon-rich atmospheres of Uranus, Neptune, Pluto, Titan, Triton

Unsaturated compounds with carbon-carbon triple bonds have been found. How many are known? What do they often have in common?

about 1000 known often toxic

O

an oil found in chrysanthenums, exhibits fungicidal activity

capicillin

OH

highly toxin compound isolated from "poison arrow" frogs in the lower Amazon basin

birth control agentOrtho-Novum, Loestrol, Ovral, etc

HO

HH

HOH

H

ethynyl estradiol

hystrionicotoxintariric acid

OH

O

fatty acid found in the seed of a plantnative to Guatemala

Here are some examples:

OH

cicutoxin

a neurotoxin found in hemlock

OH

Unsaturated compounds with carbon-carbon triple bonds have also been isolated or synthesized for use as drugs. Here are some examples:

H2N

O

NH O

parsalmide

perscription analgesicParsal, Sinovial

C4H2

LG Ch 13 p 6

pargyline

N

antihypertensive drugEudatin, Supirdyl

dynemicin A

anticancer drug - derivatives have been made which are more potent, more selective, easier to synthesize

NHOH

OH O

O

OH

OH

O

OH

O

O

What is the most useful alkyne in industry?

acetylene HC CH used in cutting and welding torches

How is it made?

CaO + 3 C

1) heat limestone and carbon to 2000oC, then add water

Ca+2 + COH2O

HC CH Ca(OH)2+

2) heat methane to 1500oC for 0.01 sec

CH4 + O2

How is it stored?

can't be stored as a liquid under pressure - heat or shock cause it to decompose to carbon and hydrogen and explode

stored dissolved in acetone under moderate pressure, absorbed onto firebrick

What is polyacetylene? What is it used for?

long chain of single, double bonds

when treated with iodine, first polymer to conduct electricity - nearly as good as silver - 2000 Nobel Prize

used for foil packaging for computer parts (dissipates static charge), light-weight batteries

What happens when it burns?

HC CH + 5 O2 4 CO2 + 2 H2O

6000o F - hottest flame from a commonly available organic gas

2

HC CH

yellow, smoky flame in air, clear blue flame in pure oxygen

made from acetylene

LG Ch 13 p 7

III. Physical Properties of Alkynes

Give the following properties of alkynes:

polarity

water solubility

density

melting and boiling points

flammability

storage issues

odor

state of matter

nonpolar

not soluble

less dense than water - will float

similar to alkenes and alkanes of the same MW

highly flammable

polymerizes easily, sometimes explosively

strong, pungent odor

2-4 C's - gas; 5-10 C's - liquid; 10+ C's - solid

IV. Spectroscopy of Alkynes

What two bonds do alkynes have that alkanes do not?

C C

C H on C C

2200-2100 cm-1

around 3300 cm-1

IR Spectroscopy

How do the IR spectra of terminal alkynes differ from those of internal alkynes?

internal - no C-H on C---C, no C---C (no dipole)

looks like an alkane

C C

C H on C C

CC

C

CC

CCH

small dipolesmall peak

no dipoleno peak

LG Ch 13 p 8

How does the carbon-carbon triple bond band compare to a double bond band?

C---C 2200-2100 C=C 1680-1620 higher frequency - stronger bond

How does the triple bond in an alkyne compare to the triple bond in a nitrile?

C---C vs C---N C---N is much deeper, absorbs more light (more polar)

How does the C-H on an alkyne compare to the C-H on an alkene?

C-H on C---C 3100-3000 higher frequency, separate from C-H deeper

1H NMR Spectroscopy

Where do H's on terminal alkynes appear?

1.7 - 3.1 ppm (not very useful, too many other things show up there)

Is the H on a terminal alkyne always a singlet?

no - can be split by H's farther away than one C because of the pi bonds (long range coupling), but the coupling constants are very small (two peaks of doublet very close together)

13C NMR Spectroscopy

Where do C's in the triple bond of an alkyne appear? 65-85 ppm (not very useful either)

V. Nomenclature of Alkynes

Common names

Give common names of the following compounds.

methylacetylene

diisopropylacetylene

ethylmethylacetylene

phenylacetylene

cyclopentylacetylene

cyclopropylmethylacetylene

C-H on C=C ~3300 lower frequency, left side of C-H small

LG Ch 13 p 9

IUPAC names

What steps should you follow when naming an alkyne?

1.

2.

3.

4.

determine stereochemistry, if any

choose the principle chain and name it

number the principle chain

name and order the substituents

What are the priorities for choosing the principle chain?

1. must contain the C---C, any C=C (all, if possible)

2. longest

3. most substituents

How are the names formed?

one C---C: change "ane" to "yne", put # in front

1-hexyne

two or more C---C: change "ane" to "yne", add "adi" or "atri" before it, put # in front

C---C and C=C: change "ane" to "en" then add "yne", put C=C # in front, C---C inside

1,4-pentadiyne 4-hexen-1-yne

What are the priorities for numbering the principle chain?

1. C---C or C=C, whichever comes first

2. if numbers are the same, C=C

3. first substituent

4. alphabet

LG Ch 13 p 10

1

23

4

56

7

12

3

4

56

7

1

12

2

3

34

45

5

1

2

3

45

6

7

12

3

45

6

7

12

3 4 5

6

8

7 8

C---C over substituent

lowest # to first C---C

lowest # to whichever comes first, double or triple bond

lowest # to C=C if the numbers are equal

lowest # to first substituent

lowest # to first in alphabet if #'s are the same

How do you name a substituent containing a carbon-carbon triple bond?

add "yn" before "yl"

ethyl ethynyl

Where do you number a substituent from?

where it is connected to the ring

2-propynyl 1-propynyl

1 12

2

3

3

(no common names like vinyl and allyl as there are in alkenes)

4-(2-propynyl)-1-cyclohexene

LG Ch 13 p 11

Name the following compounds.

1-pentyne 4-methyl-2-pentyne

1,5-hexadiyne 1-penten-3-yne

3-propyl-1-hexyne 4-(2-propynyl)-1-cyclopentene

Cl

Draw the following compounds, using the correct geometry!

5-methyl-3-octyne cyclohexylacetylene

trans-3-penten-1-yne

(Z)-3-sec-butyl-3-penten-1-yne

(S)-3-chloro-1-cyclooctyne 1-cyclopentyl-1,4-pentadiyne

3-ethynyl-1-cyclohexene

or (E)-

"yne is a line"

or (Z)-3-(1-methylpropyl)-3-penten-1-yne

(notice what a big difference that C=C makes)

LG Ch 13 p 12VI. Acid-base Properties of Alkynes

How does the electronegativity of an atom affect the acidity of a H attached to it?

more EN - more acidic

How acidic would you expect a H attached to a C to be?

not very acidic at all - C has low EN

Why are alkynes more acidic than alkenes or alkanes?

hybridization: sp > sp2 > sp3

What kind of base would be required to successfully react with an alkyne?

H Na+ base +

conjugate acidacid conjugate base

Na H-base

base

pKa 26

to favor the products, pKa must be larger than 26

What is the conjugate base of an alkyne called?

acetylide ion

NaNaH H H2 pKa 35

NaNaNH2 NH H

NH3pKa 36

What would happen if an acetylide ion came in contact with an acid with a pKa less than 26?

look on chart

Na OH+ H Na+ O

pKa 16

pKa 26 products favored

How do alkyne anions react?

Na

Na

Br

Br

H + + NaBr

+ NaBr

Nu or base

strong Nu1o

3o

strong base

SN2

E2

Which is more useful? SN2 - most common way of making alkynes

What would

happen w/ a 2o?

E2 and SN2

alkanes ~50, alkenes ~44, alkynes 26

remember this number!

base acid

*

E2 is a waste of acetylide ion - better to use t-butoxide

pKa must be greater than 26

LG Ch 13 p 13

How could an SN2 reaction be used to synthesize 5-methyl-1-hexyne from acetylene?

HC CH

NaNH2HC C Na

2 steps: 1) deprotonate the alkyne to form the Nu

2) react with an alkyl halide - SN2

Br++ NaBr

What alkyl halide would be needed to make the following alkynes?

Br

Br CH3Br

How many steps would be needed to make an internal alkyne from acetylene? 4

HC CH

NaHHC C Na

CH3Br NaH

Br

Why can't you add both alkyl halides at the same time?

you would get a mixture of products

symmetrical? use 2 eq base, 2 eq RX, only one anion forms at a time

What difficulties would you encounter in making the following alkynes?

2o alkyl halide - E2 products as well as SN2

aryl halide - no SN2

Br

3o alkyl halide - E2 only

HC CH

deprotonate attack-push-off

benzyl bromide

2-bromoethylcyclopentane

propyl bromide methyl bromide

ONLY USE 1o RX!

Br

Br

Br

LG Ch 13 p 14VII. Synthesis of Alkynes by Elimination

When alkynes can't be formed by a substitution reaction, what other method is available?

elimination

HC CH Br+substitutionadd C's to a C---C that was already there

adding a C---C to C's that were already there

How can elimination be used to make C=C?

+

base

(CH3)3COK

+

What is different about making a triple bond?

need two halides; need 2 eq of base; base must be stronger - double elimination

Br Br

NaNH2

NH2

Br

Br

Br

NaNH2

Why is a stronger base needed?

elimination from an sp2 C is much more difficult

Why aren't cumulated dienes formed?

H's on sp2 C's are more acidic than H's on sp3 C's

When a terminal alkyne is formed, an addition step where water is added is necessary. Why is this?

Br

Br 2 eq NaNH2

the H is deprotonated by the base H2O

How can this reaction be used to make alkenes into alkynes?

KOH at 200oC works too

Br2

2. H2O

Br

Br2 eq NaNH2

Br

H

alkyl halide

pKa 26

LG Ch 13 p 15

VIII. Reduction of Alkynes to Form Alkenes and Alkanes

What kinds of compounds can be formed when alkynes undergo an addition reaction with hydrogen?

alkanes

cis alkenes

trans alkenesalkyne

When an alkyne is reacted with H2 and a metal catalyst, what kind of product results?

H2, Pt/C

Why can't we isolate the product of the first stage of this reaction?

it reacts with the H2, Pd/C to form an alkane

How can this reaction be used to determine the relative stability of terminal and internal alkynes?

same product; compare the !H

H2, Pt/C

H2, Pt/C!H 275 kJ/mol

!H 292 kJ/nol

Why does the alkene intermediate have cis stereochemistry? syn addition

How could you use this reaction to synthesize an alkane chain?

HC CHBr

Br

1

2

3

1

benzene ring is not affected

OK, all RX 1owon't work, one RX is 2o

What can you do to stop the reaction at the alkene?

use a less reactive catalyst - often called a "poisoned" catalyst

LG Ch 13 p 16

Lindlar catalyst = Pd/CaCO3, PbOAc, quinoline

H2

Lindlar catalystH

H

What advantage does this have over E2 reactions to make alkenes?

no constitutional isomers, no stereoisomers

What this can you tell about the reactivity of alkenes and alkynes from this reaction?

alkynes are more reactive - they will react, alkenes won't

How can a trans alkene be formed?

can't use addition of H2 - always syn addition

totally different reaction needed

Na

NH3

H

H

How is the Na here different from NaH?

no charge - Na metal = Na still has valence e-

Why can this be called a dissolving metal reaction?

sodium metal dissolves in ammonia - deep blue color

+ 2 NaNH2

Where do the H's come from?

from the ammonia

Na Na

N

HH

H

N

HH

H

What is the mechanism of this reaction?

Na

NaNH2Na

NH2Na

2

3

one product!

you do not need to be able to reproduce this mechanism

LG Ch 13 p 17

H2

Synthesize the following compounds from an alkyne.

Pd/Cor

H2

Na

NH3

Lindlar

can't do!

IX. Addition of HX and X2 to Alkynes

If HCl, HBr, or HI is added to an alkyne, what product will result?

HBrBr

HBr

Br

Which of these reactions demonstrates regioselectivity, and which side gets the halide?

terminal alkynes - halide goes to the more substituted side

What will happen if an internal alkyne is not symmetrical?

two products

Addition of HX

What will happen if a second equivalent of HX is added?

geminal dihalide results

Br Br

BrBr

HBr

HBr

doesn't exist!

H2, Lindlar

or Na, NH3

1 eq 1 eq

LG Ch 13 p 18

What happens if there is peroxide present in an HBr addition?

ROOR

HBrBr

Addition of X2

What products are formed when one or two equivalents of Br2 are added to an alkyne?

Br2

Br

Br

2 eq Br2 Br Br

Br Br

vicinal vinyl dihalide

vicinal tetrahalide

X. Hydration of Alkynes

What reagents are required to add water across a carbon-carbon triple bond?

Hg(OAc)2

H2SO4, H2OOH

O

What kind of product results? What happens to it?

enol

tautomerization - H and == change places

Which side is favored on a terminal alkyne?

HgSO4

H2SO4, H2O

OH O

more substituted side

What will happen if an internal alkyne is not symmetrical?

Hg(OAc)2

H2SO4, H2O

O

O

2 products

+

Acid- and mercury-catalyzed hydration

ROOR

HBrBr

Br

LG Ch 13 p 19

Hydroboration-oxidation of alkynes

What regioselectivity is observed when an alkyne is treated with a borane reagent?

1. BH3-THF

2. H2O2, OH-

OH O

OH goes to the lesssubstituted side

What product results when an alkyne is treated with a borane reagent?

enol ketone

What complication could occur? How do we prevent this?

C=C in the enol could react with the borane again

internal alkynes - steric hindrance of alkyne is enough

terminal alkynes - use sterically hindered borane reagent

disiamylborane

B H

B

H

dicyclohexylborane

1. disiamylborane

2. H2O2, OH-

OH O

H

aldehyde

How could the following compounds be synthesized from an alkyne?

HgSO4

O

H2SO4, H2O

O

H

1. disiamylborane

2. H2O2, OH-

LG Ch 13 p 20

XI. Oxidation of Alkynes

What happens when an alkyne reacts with KMnO4?

H2O

2 eq KMnO4

OH

OH

HO OH

HO OHO

O

diketoneWhat happens when an alkyne reacts with ozone?

O3

H2O

OH

O

OH

O

+

2 carboxylic acids

Match the following reactions and products:

KMnO4

2 eq KMnO4

H2O, NaOH

H2O, NaOH

OH

HO

H

O

O

1. Hg(OAc)2, H2O

2. NaBH4

1. BH3-THF

2. H2O2, NaOH

H2SO4, H2O

Hg(OAc)2

1. dicyclohexylborane

2. H2O2, NaOH

OH

OH

O

O

H

Summary of alkyne reactions

X

X

HC CH

XX

NaNH2

CH3Br

NaNH2

CH3Br

HC CH

HC CH

Synthesis of alkynes

NaNH2 or

KOH, 200oC

acetylene terminal alkyne

terminal alkyne internal alkyne

vicinal dihalide

geminal dihalide

alkyne

alkyneNaNH2 or

KOH, 200oC

X2

HX

Na

H2

H2

Hg(OAc)2

NH3

X

XX

HX

O

O

OH

O

KMnO4

H2O

O3

H2O

O

X2

X X

H

O

X X

X

X

Reactions of alkynes

alkane

cis alkene

trans alkene

vinyl halide or geminal dihalide

vicinal vinyl dihalide or tetrahalide

ketone

aldehyde

diketone

two carboxylic acids

Pd/C

Lindlar

H2SO4, H2O

1. disiamyl borane

2. H2O2, NaOH

2

alkyne

alkyne

alkyne

alkyne

alkyne

alkyne

alkyne

alkyne

alkyne

LG Ch 13 p 21